The present invention relates to a heat roller fixing device that is used in an image recording apparatus such as an electrophotographic copying machine for the purpose of fixing toner images formed on a recording paper.
In an electrophotographic copying machine or the like, toner images are formed on an electrophotographic photoreceptor and then the toner images thus formed are transferred onto a recording paper so that they adhere thereon electrostatically, and then are fixed by a heat roller fixing device onto the recording paper which is then ejected out of the copying machine.
A heat roller fixing device is composed of a heat roller having therein a built-in heater thereby controlling and keeping the surface of the roller at the temperature optimum for fusing and fixing toners and of a pressure roller that contacts the heat roller with pressure. The pressure roller is equipped, on its surface or in the vicinity of its surface, with an elastic layer such as a rubber layer or the like, and thereby a nip portion is formed between the heat roller and the pressure roller which are in contact with pressure (a nip portion is an area where both rollers contact each other). Fixing of toners on a recording paper is mainly carried out while the recording paper passes through the nip portion. Both FIG. 4 (a) and FIG. 4 (b) show nip portion N formed between heat roller 11 and pressure roller 12 which are in pressure-contact. As shown in FIG. 4 (b), shapes of rollers are usually determined so that nip width N.sub.A at the central portion of the roller is slightly smaller than that N.sub.B in the vicinity of each end portion of the roller. When a recording paper is nipped between the heating roller and the pressure roller to be transported and fixed therewith, such forces as shown by arrows in the FIG. 4(b) are exerted on the recording paper by the rollers due to an hourglass-shaped nip portion shown as a hatched area, and these forces are summed up to form a pair of tensile forces in lateral direction of the recording paper in addition to conveying force. The heat roller and the pressure roller of the fixing device are generally shaped in such forms as to generate the aforesaid pair of tensile forces for the purpose of preventing the recording paper from being creased during fixing and ejection processes.
In the heat roller fixing device stated above, heat roller 11 is generally arranged to rotate only for the period of fixing, and in a heat roller fixing device of a certain type, pressure roller 12 is always in contact with heat roller 11, while in that of other type, pressure roller 12 contacts only for the period of fixing. However, both of them, regardless of their types, have had following the disadvantages.
Namely, in a copying machine provided with an automatic document feeder (ADF), for example, when plural sets of copies need to be made for multiple documents, same size copies in the quantity of 100 sheets or more are sometimes made continuously. Immediately after the completion of such continuous copying for the multiple sheets of the same size, copying on the recording paper that is larger in size than the previous one is newly needed frequently. In such a case, copied images are sometimes disturbed, resulting in defective fixed copied images.